Infrasound from Tornados: Theory, Measurement, and Prospects for Their Use in Early Warning Systems
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Infrasound from Tornados: Theory, Measurement, and Prospects for Their Use in Early Warning Systems Carrick Talmadge Tornados may produce a low-frequency signature that could be used in automatic warning systems. Postal: National Center for Physical Acoustics Introduction University of Mississippi Infrasound is propagating sound waves with frequencies below the range of human University, MS 38677 hearing. A practical frequency range for infrasound that will propagate over long USA distances is 0.01-20 Hz. The lower frequency cut off for propagating infrasound Email: arises because the buoyancy of parcels of air becomes comparable to the pressure [email protected] gradient forces of acoustic waves at sufficiently low frequencies. The precise -fre quency for this cut off occurs at the Brunt–Väisälä frequency (Stull, 1995), the Roger Waxler exact value of which depends on details of the vertical profile of the atmosphere. Postal: One of the more famous examples of long-range propagating infrasound is the ex- National Center for Physical Acoustics plosive eruption of Krakatoa Volcano in Indonesia. Audible sounds were heard as University of Mississippi far away as 5,000 km, and low-frequency infrasound signals with periods greater University, MS 38677 than one minute were propagated around the Earth at least seven full times (e.g., USA Fee and Matoza, 2013). Email: [email protected] Figure 1. Annual average death rate from tornados. Also shown are the geographical re- gions referred to in the text. Note that these geographical regions correspond approximately to tornado climatological zones rather than to traditional geographical regions. Adapted from original figure by The National Oceanic and Atmospheric Administration (NOAA). Because infrasound propagates for long ranges, it can be used for hazard moni- toring. For volcanic warnings, atmospheric blasts can be detected by infrasound stations even when the peak of the volcano is obscured by clouds so that optical observations are not possible. Another important infrasound source is hurricanes, which produce characteristic tonelike signals called “microbaroms” produced by All rights reserved. ©2016 Acoustical Society of America. volume 12, issue 1 | Spring 2016 | Acoustics Today | 43 Infrasound from Tornadoes the nonlinear interaction between ocean waves and the at- mosphere (Waxler and Gilbert, 2006). It was reported by Raveloson et al. (2012) that infrasound signals from the 2011 Tohoku-Oki, Japan, earthquake could have been used as an early warning of the impending tsunami. Tornados represent one of the most common natural haz- ards posed in the United States. Within any given year, on average, some 800 tornados will occur within the United States east of the Rockies, resulting in 80 deaths and 1,500 injuries [National Oceanic and Atmospheric Administra- tion (NOAA) National Severe Storms Laboratory]. In spite of the mystique associated with “Tornado Alley” (typically listed as the states of Oklahoma, Kansas, and Nebraska as Figure 2. Radar reflectivity for the Texas supercell over Wichita Falls, well as adjoining areas from neighboring states), southern TX on April 11,1979. The location of the mesocyclone is shown by the states, including Mississippi, remain a primary target for circle, which is oriented over the hook region of the storm. Obtained tornadic activity. These regions are shown in Figure 1. from NOAA, Don Burgess, OSF, and Vanessa Ezekowitz. Even with recent advances in Doppler radar technology and other early warning systems, tornados in the central regions our recent tornadic thunderstorm measurements in Okla- of the United States remain a significant risk for injury or homa (Frazier et al., 2014). Finally, we show that there are loss of life. Augmenting existing detection arrays with infra- characteristic signals that seem to be present only when tor- sound/low-frequency arrays offers a possible new method nados have touched down. It is possible that these character- for improving the safety of people living in high-risk tor- istic signals could be used to augment existing early warning nado areas. systems and lead to an improvement in safety for people at risk from tornadic thunderstorms. We focus here on the possible generation of infrasound from tornados and detection of these waves on a regional scale Tornado Genesis (i.e., over distances up to 100 km from the source). For this There are two generally accepted patterns for energetic tor- distance scale, the main influences on infrasound propaga- nado generation (reviewed in Bluestein, 1999). The first is tion are the distance of the source and the effective vertical associated with supercell thunderstorms (storms that are atmospheric sound-speed profile in the direction of prop- large, typically 15 km wide by 15 km tall) that have a deep, agation (e.g., Attenborough et al., 2006). In a (theoretical) continuously rotating updraft known as the mesocyclone. atmosphere with a constant vertical sound speed, sound Tornados from supercells are the most studied class of tor- intensity decreases as the inverse square of the propagation nados due to the relative predictability of where a tornado distance. For “downwind” propagation—known as “ducted” will appear within the supercell (the “hook region” in Figure propagation—sound intensity decreases inversely with dis- 2). These are also studied in part because of the visibility of tance. Hence it is important for long-distance propagation supercells, which often have localized precipitation zones measurements to place the arrays downwind from the infra- separated from the mesocyclone (a large region of rotation sound source when possible. within a thunderstorm) and because of the geographical ac- cessibility of the Great Plains region where many of this type In this paper, we start by discussing the mechanisms for tor- of storm occur. Supercells are noted for producing the most nado genesis. We then review the hazards associated with intense and dangerous tornados, with nearly half of all fa- tornados, noting that while the Great Plains (a grassland talities occurring from these intense storms. However, even prairie east of the Rocky Mountains that extends from the with their relatively well-defined structure, current radar al- southern US border into Canada) have received much of the gorithms achieve less than a 50% probability of detection for attention for tornadic research, there is a substantial safety acceptable levels of a false alarm rate (Mitchell, 1998). risk from tornados in the “US Mid-South” (Figure 1). We then discuss evidence that tornado vortices produce infra- The other mechanism for tornado genesis is simply referred sound and low-frequency sound. We review the results from to as “non-supercell.” Typically, these tornados are associ- 44 | Acoustics Today | Spring 2016 ated with less organized storm systems, in which precipita- tion bands shroud the region of formation of the tornado. Tornados generated by this mechanism tend to be weaker, but they are very dangerous due to their lack of predictabil- ity and the poor visibility associated with the phenomenon. Roughly one half of the storms in the Southeastern United States are of this variety, with almost 100% of the tornados spawned in Florida being non-supercell in origin (Kelly et al., 1978). Tornado Alley Tornados in the US Plains States represent an all too famil- iar threat to life, health, and property. What is not typically well appreciated is that the area that has the largest threat for loss of life is the United States Mid-South. Contributors Figure 3. Histogram of tornadic events for (a) the "Mid-South" geo- to the higher mortality rate include rain-wrapped thunder- graphical region and (b) the “South-Central” geographical region. storms, which obscure tornados within the storm cell as well The states used for each region are shown by their state abbreviations. more convoluted terrain and the presence of tall forests that The green shaded area is for all tornadoes, and the magenta area is obscure the horizon. Without advanced warning from the for just violent (EF3-EF5) tornados. emergency broadcast system that provides alerts to oncom- ing tornados, people who live in rural areas in these states are subject to the whims of nature. In spite of the South-Central region’s deserved reputation In the Great Plains, low-precipitation supercells are a rela- for having a high rate of tornados, we see that the Mid-South tively common occurrence (Bluestein, 1999). Because of the has nearly identical rates for tornados as the South-Central, lack of significant precipitation, very clear views of tornadic and both the East-North-Central (Figure 1) and Mid-South activity are usually possible, making them ideal for the study regions suffer higher fatality rates than does the South-Cen- of tornados. The relatively smooth terrain and lack of tall tral region. Lower visibility due to terrain, presence of trees, forests also allow views to the horizon in many cases. and a higher frequency of rain-wrapped tornados are likely contributors to these higher fatality rates. Possibly the fact In Figure 3 we show histograms of the total number of tor- that Plains States tornados and the meteorological factors nados for each day over the period 1950–2009 summed over associated with their genesis are well-studied has reduced the South-Central and over the Mid-South regions (as de- the risk of fatality associated with South-Central tornados. fined in Figure 1). These figures show that the tornado sea- son is roughly concentrated in the band from days 80–180 Infrasound from Tornados of the year (mid-March through the end of June of each Since the early 1970s, convective systems have been known year). For the Mid-South, there is strong activity from Janu- to be sources of infrasound (e.g., Bedard and Georges, 2000). ary through March but also late season activity in November The typical frequency band for these sounds is about 0.017 and December.